I agree that the hair dryer should work well to test the engine/generator.
First check it with a voltmeter. You should have 120 volts +/- about 8 volts. Under load, then the voltage might drop to the 115V range. Then plug in the hair dryer, it might drop to the 115 or 112 volts, but really should be near 118V.
Then you can feel safe adding the A/C units that are so much more expensive to replace. ..
All in all, you should be able to run a coffee maker, electric heater, hair dryer and 2 A/C units all at once, putting a very large load on the generator. With all those loads running, if you are getting 110 - 118 volts, then it is fine!
Fred.
